MR B. STEWART ON RADIANT HEAT. 13 
readily than heat of high temperature, concluded that there are a few rays for 
which rock-salt is opaque.* 
We conclude, therefore, that every body which sifts heat in its passage through 
its substance, is more opaque with regard to heat radiated by a thin slice of its 
own substance, than it is with regard to ordinary heat. 
19. This conclusion may be also stated thus: We have before proved (Art. 12.) 
that the radiation of a thin slice of any substance equals its absorption; we now 
add, that the heat radiated is the same as that absorbed, with regard to quality 
as well as quantity. 
For this expresses the fact, that substances which sift heat are likewise opaque 
with respect to heat radiated by themselves. For, since the heat which they ab- 
sorb is manifestly that kind of heat for which they are opaque, if the descrip- 
tion of heat radiated is the same as that absorbed, then they will also be opaque 
with respect to heat radiated by themselves. Considering, therefore, the heat of 
any temperature to consist of heterogeneous rays, we may state the law thus: 
« The absorption of a plate equals its radiation, and that for every description of 
heat.” 
20. A more rigid demonstration may be given thus :—Let 
AB, BC be two contiguous, equal, and similar plates in the inte- : 
rior of a substance of indefinite extent, kept at a uniform tempe- ———~+——— 
rature. The accumulated radiation from the interior impinges | 
on the upper surface of the upper plate; let us take that portion © 
of it which falls on the particle A, in the direction DA. This ray, in passing 
from A to B will have been partly absorbed by the substance between A and B; 
but the radiation of the upper plate being equal to its absorption (since its tem- 
perature remains the same), the ray will have been just as much recruited by the 
united radiation of the particles between A and B, as it was diminished in inten- 
sity by their absorption. It will therefore reach B with the same ‘ntensity it 
had at A. But the quality of tne ray at B will also be the same as its quality at 
A. For, if it were different, then either a greater or a less proportion would be ab- 
sorbed in its passage from B to C, than was absorbed of the equally intense ray 
at A, in its passage between A and B. The amount of heat absorbed by the par- 
2 


* To take a numerical example, let us suppose the heat from a single plate of rock-salt to 
be =1, then the heat from a plate four times the thickness, or (which is the same thing) the heat 
from four single plates, one behind another, should be nearly four times as much, or = 4 (if we sup- 
pose the heat from each of these four plates to be readily passed by the plates between it and the 
pile), but the heat from the fourfold plate, instead of being four times as much, is not double of the 
heat from the single plate; hence, the heat from any of the interior plates of the compound plate is 
passed with great loss, by the plates between it and the pile. Now, since the absorption of a plate 
equals its radiation, the reason why the fourfold plate scarcely radiates twice so much as the single 
one is, that it scarcely absorbs twice as much ; and this again is due to the fact, that the heat after 
it has passed the first plate of the fourfold plate has become sifted, and passes with little diminution 
of intensity through the other three plates. 
VOL. XXII. PART I. D 
